Welding defects of spiral welded steel pipes

In the submerged arc welding zone of spiral welded steel pipes, certain defects are particularly prone to occur, such as pores, hot cracks, and undercuts. This article provides an overview of these common welding defects and their preventive measures.

Pores

Pores are frequently found at the center of the weld bead. The primary cause is hydrogen trapped within the weld metal, forming bubbles. To eliminate this defect, it's crucial to remove rust, oil, water, and moisture from the welding wire and weld area. Additionally, the flux must be thoroughly dried to eliminate moisture. Increasing the current, reducing the welding speed, and slowing the solidification rate of the molten metal are also effective in reducing the occurrence of pores.

Preventive Measures:

Pores and slag inclusions due to contamination of the welding wire or flux tend to be continuous. If pores or slag inclusions are detected throughout the weld, the first step is to check the purity of the welding wire, replace the flux, and inspect for oil stains or rust on the wire surface. If the pre-welded area shows a honeycomb-shaped gas connection, it may be necessary to increase the current of wire 1 and wire 2 or reduce the welding speed, allowing more time for the pores to escape the molten pool.

Sulfur Cracking (Cracks Caused by Sulfur)

Sulfur cracking occurs when welding plates with strong sulfur segregation zones, particularly in soft boiling steel. This happens because sulfides in the sulfur segregation zone enter the weld metal. To prevent sulfur cracking, using semi-killed or killed steel with minimal sulfur segregation is effective. Additionally, ensuring the weld surface and flux are clean and dry is essential.

Thermal Cracks

Thermal cracks are common in submerged arc welding, especially in the weld bead at the arc start and end points. To prevent this, backing plates are often installed at these points. At the end of plate coil butt welding, reversing the spiral welded pipe and welding it into the stack can also help. Thermal cracks are more likely when the weld stress is high or the Si content in the weld metal is elevated.

Preventive Measures: If cracks are detected, pickling samples or repair welding gouging is necessary to confirm whether the cracks are internal or external. Cracks in precision welding are usually external. Prevention involves reducing the welding line energy by lowering the welding current and arc voltage, minimizing external welding eccentricity, and increasing the dry elongation of the external welding wire.

Weld Slag Inclusion

Weld slag inclusion occurs when part of the welding slag remains trapped in the weld metal.

Poor Welding

Penetration Insufficient penetration occurs when the metal overlap of the inner and outer welds is inadequate, leading to incomplete welding. This condition is referred to as insufficient weld penetration.

Undercut

An undercut is a V-shaped groove that forms along the edge of the weld. It occurs when welding speed, current, or voltage are not set correctly, with excessively high welding speed being a common cause of undercut defects.

By understanding these common defects and implementing preventive measures, the quality and reliability of spiral welded steel pipes can be significantly improved.